A liquid crystal display device includes a liquid crystal panel 22 and a backlight unit 21 located behind the panel and provided with a light source FL (see FIG. 42). In each of pixels of the liquid crystal panel 22, liquid crystal is driven by video signals, light emitted from the backlight unit 21 is transmitted therethrough, and images are displayed on the liquid crystal panel 22. In general, a cold cathode fluorescent lamp (CCFL) is often used as the light source FL of the backlight unit 21, and a discharge lamp lighting device for lighting control of the lamp is required. An example of a method of dimming the CCFL in the discharge lamp lighting device is a burst dimming (PWM dimming) method.
The burst dimming method is a so-called intermittent lighting operation in which dimming is performed by causing the light source to blink in cycles while changing a time ratio between a light-on period and a light-off period thereof. Therefore, an appropriate selection of the blinking cycle makes it possible to set a dimming ratio to 100 to 1. For this reason, the burst dimming method has been employed for backlight control in many liquid crystal display devices. As disclosed in Patent Document 1, this burst dimming method is also applied to means for improving a liquid crystal display device to fix moving image display with indistinct outlines (called blurred moving images or edge blurs), which is incurred due to its own poorer moving image display performance than that of a CRT.
In recent years, particularly in the field of liquid crystal display devices, demands for a size increase, higher luminance, and uniformity of a screen have brought about tendencies to increase the number of lamps employed in one set of device, and to make higher tube voltages of discharge lamps used in the screen. Even a CCFL used for backlight in a 32-inch size has a tube voltage of about 1 kV (rms). For this reason, effects of a high-impedance load and parasitic capacitance between the CCFL and a casing is not negligible, which leads to a problem that a current leakage to the casing causes a deviation in the luminance distribution, thereby making luminance of the lamp nonuniform.
Accordingly, it is conceivable to use a hot cathode fluorescent lamp (HCFL) having a higher output and lower tube voltage than the CCFL does. The use of the HCFL has advantages to drastically reduce the number of lamps and to reduce the number of lighting circuits as compared to the use of the CCFL. Moreover, since the tube voltage is low, the effect of the parasitic capacitance between the HCFL and the casing is small and a deviation in luminance is also reduced. Further, due to low noise, an effect on peripheral circuits such as a liquid crystal panel is also small.
FIG. 43 shows a discharge lamp lighting device disclosed in Patent Document 2, which switches light-on periods and light-off periods by opening and short-circuiting a switch SW2 between filaments of a hot cathode discharge lamp FL. Under control of a converter control circuit 45, the voltage of an output from a DC power source 41 is converted by a DC-DC converter 44, and the output at the converted voltage is again converted into high-frequency power by a high-frequency inverter 47, and supplied to the hot cathode discharge lamp FL. The switch SW2 of a dimming circuit 49 is connected between the filaments of the hot cathode discharge lamp FL so as to be in parallel with the hot cathode discharge lamp FL.
Short-circuiting and opening operations of the switch SW2 are controlled by a dimming control circuit 46. A preheating current flows to the filaments of the hot cathode discharge lamp FL by an output from the high-frequency inverter 47 when the switch SW2 is short-circuited, while a lamp current flows between the filaments of the hot cathode discharge lamp FL by the output from the high-frequency inverter 47 when the switch SW2 is opened. For this reason, the hot cathode discharge lamp FL is lighted off when the switch SW2 is short-circuited, while the hot cathode discharge lamp FL is lighted on when the switch SW2 is opened. Accordingly, a light output can be controlled by adjusting a time ratio between a light-off period and a light-on period.
An output from the hot cathode discharge lamp FL is monitored by an optical sensor S and is subjected to feedback control by a CPU 43 so as to maintain a dimming state set by an operating unit 42. In addition, an output voltage from the DC-DC converter 44 during the light-off period is variable by the converter control circuit 45 in accordance with the dimming state detected by a dimming state detection circuit 48, whereby a filament voltage during the light-off period is variable in accordance with the dimming state.
In this lighting device, waveforms of the filament preheating current flowing in the light-off period and of the lamp current flowing in the light-on period each vary in accordance with the dimming state as shown in FIGS. 44(a) to (c). Amplitude of the filament current is variably controlled in accordance with the dimming state so as to make the filament temperature appropriate according to time ratios between light-off periods Ta, Tb and Tc in which the filament preheating current flows, and, light-on periods ta, tb and tc in which the lamp current flows.
On the other hand, Patent Document 3 discloses a discharge lamp lighting device in which a semiconductor switch 54 cuts off a signal from variable pulse width modulator means 84 to turn a lamp current on and off, thereby expanding a dimming range (see FIG. 1). In this discharge lamp lighting device, constant filament voltage means 12 supplies preheating voltages ranging from 0 to 10 V to electrodes 26 and 28 of a lamp 10 via a transformer 16.
The technique according to Patent Document 2 can control the optical output by adjusting the time ratio between the light-off period and the light-on period. However, the technique imposes large stresses on the switch SW2 due to an inrush current at the time of disconnecting a high-frequency current or at a moment when the switch SW2 is turned on, a starting voltage applied to the lamp at a moment when the switch SW2 is turned off, and the like. Moreover, since a voltage applied to and a current flowing to the filaments are large when the switch SW2 is turned on, the lamp life is deteriorated. Further, it is not possible to supply the preheating current to the filaments during lighting.
In the case of a liquid crystal display backlight device, a burst dimming frequency has a repetitive waveform around several hundred hertz. Accordingly, an electrode temperature is influenced by an average value of the preheating current supplied to the filaments. For this reason, if there is a period when no preheating current is supplied, it is necessary to supply the preheating current at a larger peak value than in the case of constantly supplying the preheating current, in order to ensure the electrode temperature, thereby causing circuit stresses. In addition, a starting voltage also has to be high enough for the discharge lamp to transition from the light-off state to the light-on state, thereby causing circuit stresses as well. Particularly in the technique according to Patent Document 2, the preheating current is completely cut off at the time of continuous lighting, and thus the electrode is locally heated. The entire electrode needs to be heated at the transition from the light-off state to the light-on state in response to switching from continuous lighting to burst dimming. This causes a problem that the peak value of the starting voltage becomes high.
Furthermore, if an inverter circuit for lighting and an inverter circuit for preheating are separately provided for supplying the lamp current and the preheating current independently, the configuration is so complicated that control thereof is also complicated. In the case of an integrated type (a type supplying the preheating current from a secondary coil of a ballasting inductor) that can simultaneously provide the lamp current and the preheating current by oscillation of an inverter circuit, oscillation of the inverter circuit is turned on and off in order to turn the lamp current on and off. In this case, when the oscillation is turned off, the preheating current is also stopped. The preheating current can be continuously supplied by keeping the oscillation of the inverter circuit, i.e., for example, by alternately repeating a full light-on state and a dimmed state. However, in this case, a lower dimming limit cannot be lowered enough.
On the other hand, in the technique according to Patent Document 3, it is possible to turn the lamp current on and off by using burst signal from variable pulse width modulator means 84 while continuously supplying the preheating current. Nevertheless, the circuit is complicated as the semiconductor switch 54 for turning off the lamp current is necessary. Moreover, it is necessary to provide a transformer 40 and semiconductor switches 42 and 44 for supplying the lamp current in addition to the transformer 16 and semiconductor switches 18 and 20 for supplying the preheating current, which makes the circuit further complicated and leads to an increase in circuit costs.
The present invention has been made in view of the above-mentioned problems and an object thereof is to provide a discharge lamp lighting device, an illumination device, and a liquid crystal display device, which are capable of dimming to a low level, maintaining a necessary electrode temperature, reducing circuit stresses, and increasing a lamp life.
Patent Document 1: JP-A 2006-53520
Patent Document 2: JP-A 8-106987
Patent Document 3: U.S. Pat. No. 4,998,046